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intro_to_geo_8-1

intro_to_geo_8-1 - Magma and igneous rocks cont...

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Magma and igneous rocks cont. Classification Igneous rocks are classified on the basis of grain size phaneritic, aphanitic, or glassy mineral composition felsic, intermediate, mafic, or ultramafic) Grain size and composition should be known for the standard six rock types granite, diorite, gabbro, rhyolite, andesite, and basalt, plus peridotite Grain size and cooling Slow rate : Large, thermodynamically stable crystals form when a melt cools and crystallizes gradually Fast rate : Rapidly cooling and crystallizing melts do not leave sufficient time for large crystals to become organized Factors controlling cooling rate Magmas that cool deep within Earth cool more slowly than those that cool near the surface Large, globular bodies of magma cool more slowly than those that are smaller and sheetlike Cool groundwater can absorb heat from magma and transport the heat away from the source, greatly increasing the cooling rate Porphyritic texture and cooling history
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